1. Field of the Invention
Aspects of the present invention relate to a fixing device that fuses an image transferred to a print medium and an image forming apparatus having the same.
2. Description of the Related Art
In general, an electrophotographic image forming apparatus, such as, a laser printer, a photo-copier, a facsimile machine, and a multifunctional produce, prints an image by scanning light onto a photosensitive medium that is charged with a predetermined electric potential, in order to form an electrostatic latent image. The latent image is developed with a predetermined color toner, transferred to the print medium, and then fixed to the print medium. A fixing device is provided along a print path of the image forming apparatus.
Referring to FIG. 1, a conventional fixing device includes a heating roller 3, a heating lamp 1 disposed inside the heating roller 3, a pressing roller 5 facing the heating roller 3, and a temperature sensor 7. The pressing roller 5 is elastically biased toward the heating roller 3 by an elastic member, to form a fixing nip position N1.
The heating roller 3 includes a core pipe 3a formed of metal, and an elastic layer 3b disposed on the core pipe 3a. Radiant energy from the heating lamp 1 is converted into heat by a light-heat converting layer (not shown) disposed on an inner surface of the first core pipe 3a, to heat the core pipe 3a. Also, the elastic layer 3b is heated by thermal conduction to a predetermined fixing temperature.
The temperature sensor 7 can contact, or be adjacent to, the heating roller 3 and measures a surface temperature of the elastic layer 3a. Accordingly, power supplied to the heating lamp 1 can be adjusted, based on the surface temperature measured by the temperature sensor 7.
The pressing roller 5 includes a core pipe 5a formed of metal, and an elastic layer 5b disposed thereon. The elastic layer 5b has less elasticity than the elastic layer 3b, and the elastic layer 5b is distorted when pressed against the pressing roller 5. When a print medium 9 having a toner image 9a passes through the fixing nip position N1, the toner image is heated, pressed, and thereby fixed to the print medium 9.
To increase the speed of the image forming apparatus employing the fixing device, the external diameters of the heating roller and the pressing roller can be increased, or the thickness of the elastic layers 3b and 5b can be increased. Such modifications increase a fixing time of the print medium in the fixing nip position N1, by enlarging the width of the fixing nip position N1, in order to compensate for an increased speed of the image forming apparatus. Such modifications prevent a fixing quality deterioration caused by a decrease of the fixing time of the print medium.
However, there is a practical limit to the enlargement of the external diameters of the heating roller and the pressing roller, because such enlargements increase the total size of the image forming apparatus. Such enlargements also increase a warm-up period of the rollers and increase manufacturing costs.
To solve the warm-up delay, a conventional fixing device employing a local heating method has been disclosed. This fixing device reduces a warm-up time, by disposing the heat source to concentrate heat on the fixing nip position. However, if the print medium is not transported, a pressing member contacting the fixing nip position is damaged, due to overheating of the fixing nip position.